fix(xam): XamUserGetSigninState returns SignedInLocally=1 for user 0

Mirrors canary xam_user.cc:90-101. User 0 returns 1 (SignedInLocally),
all other indices return 0. Replaces stub_return_zero registration that
was reaching guest-side branches looking up signin state.

Tests: 599 -> 600.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-kernel/src/xam.rs

@@ -45,7 +45,7 @@ pub fn register_exports(state: &mut KernelState) {
     // User
     state.register_export(Xam, 0x020A, "XamUserGetXUID", xam_user_get_xuid);
     state.register_export(Xam, 0x020E, "XamUserGetName", xam_user_get_name);
-    state.register_export(Xam, 0x0210, "XamUserGetSigninState", stub_return_zero);
+    state.register_export(Xam, 0x0210, "XamUserGetSigninState", xam_user_get_signin_state);
     state.register_export(Xam, 0x0219, "XamUserReadProfileSettings", xam_user_read_profile_settings);
     state.register_export(Xam, 0x021A, "XamUserWriteProfileSettings", stub_success);
 
@@ -329,6 +329,12 @@ fn xam_user_read_profile_settings(ctx: &mut PpcContext, _mem: &GuestMemory, _sta
     ctx.gpr[3] = 0x0000_048B; // ERROR_NOT_FOUND
 }
 
+fn xam_user_get_signin_state(ctx: &mut PpcContext, _mem: &GuestMemory, _state: &mut KernelState) {
+    // r3 = user_index
+    let user_index = ctx.gpr[3] as u32;
+    ctx.gpr[3] = if user_index == 0 { 1 } else { 0 };
+}
+
 // ===== System =====
 
 fn xam_get_execution_id(ctx: &mut PpcContext, mem: &GuestMemory, state: &mut KernelState) {
@@ -572,6 +578,22 @@ mod tests {
         assert_eq!(ctx.gpr[3], 8);
     }
 
+    #[test]
+    fn xam_user_get_signin_state_user0_signed_in_locally() {
+        let (mut ctx, mem, mut state) = fresh();
+        ctx.gpr[3] = 0;
+        xam_user_get_signin_state(&mut ctx, &mem, &mut state);
+        assert_eq!(ctx.gpr[3], 1);
+
+        ctx.gpr[3] = 1;
+        xam_user_get_signin_state(&mut ctx, &mem, &mut state);
+        assert_eq!(ctx.gpr[3], 0);
+
+        ctx.gpr[3] = 4;
+        xam_user_get_signin_state(&mut ctx, &mem, &mut state);
+        assert_eq!(ctx.gpr[3], 0);
+    }
+
     fn drain_notifications(state: &mut KernelState, mem: &GuestMemory, handle: u32) -> Vec<(u32, u32)> {
         let id_ptr = SCRATCH_BASE + 0x100;
         let param_ptr = SCRATCH_BASE + 0x104;